1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus capable of maintaining a gap between a donor roller and a magnetic roller and a gap between a donor roller and a photosensitive body to a predetermined gap.
2. Description of the Related Art
An image forming apparatus refers to an apparatus, in which an electrostatic latent image corresponding to a desired image is formed on a photosensitive body by an exposure unit, such as, e.g., a laser scanning unit. The electrostatic latent image is developed into a visible image by the use of developer, and the visible image is transferred and fused onto a printing medium. The image forming apparatus includes a developing unit which supplies the developer to the photosensitive body and develops the electrostatic latent image into a visible image.
For example, FIG. 1 illustrates a developing unit generally in use in an image forming apparatus, and FIG. 2 is a perspective view showing the relevant portions of the developing unit depicted in FIG. 1. As shown in FIGS. 1 and 2, a developing unit 200 includes a photosensitive body 220, a magnetic roller 240 to supply a developer to the photosensitive body 220, and a donor roller 230 mounted between the magnetic roller 240 and the photosensitive body 220.
In a one-component developing system, either using an insulating developer or a conductive developer, the donor roller 230 serves to make an image of high quality. In a two-component developing system using a magnetic carrier and a toner, the donor roller 230 serves to prevent the carrier of the magnetic roller 240 from moving to the photosensitive body 220.
To obtain a desired image quality, the developing unit 200 should maintain each of the gap between the donor roller 230 and the photosensitive body 220 (i.e., the developing gap) and the gap between the donor roller 230 and the magnetic roller 240 to be uniform. For example, FIG. 2 illustrates a holding member 281 formed with a pair of through-holes 282 and 283, through which the rotating shaft 231 of the donor roller and the rotating shaft 241 of the magnetic roller are respectively received and supported. This holding member 281 maintains uniform gaps between the donor roller 230 and the magnetic roller 240.
Also provided is an elastic member 285, which elastically supports one side of the holding member 281 and pushes the holding member 281 toward the photosensitive body 220. By the elastic bias imparted by the elastic member 285 on the holding member 281 toward the photosensitive body 220, the gap between the donor roller 230 and the photosensitive body 220 are maintained.
An image forming apparatus with the above described configuration, however, unfortunately, relies on the precisely formed through-holes and/or the rotating shafts to maintain the gaps, and thus often fails to maintain the gaps to the desired preset gaps.